1. Field of the Invention
The invention relates to an automatic transmission to be mounted in a vehicle or the like.
2. Description of the Related Art
In addition to a plurality of gears which form a gear train for transmitting power, an automatic transmission is also provided with a hydraulic pressure supply unit that switches gear ratios using hydraulic pressure, and a control unit which controls a solenoid valve of the hydraulic pressure supply unit based on the running state of the vehicle, for example. Here, the hydraulic pressure supply unit is arranged inside a case of the automatic transmission, while the control unit is usually arranged outside of the case because it houses many electronic components.
Distancing the hydraulic pressure supply unit and the control unit from one another by having one located inside the case and the other located outside the case in this way increases the length of the signal wire that electrically connects the two. As the length of the signal wire increases, the reliability of the control signal decreases due to the weight of the noise and the like, for example.
It is therefore conceivable that this problem could be avoided by arranging the control unit inside the case of the automatic transmission together with the hydraulic pressure supply unit so that the signal wire which connects the hydraulic pressure supply unit to the control unit can be shorter.
The problem with this structure, however, is that when the control unit itself generates heat, the temperature of the elements in the control unit may exceed the allowable temperature due to radiant heat from the gear train and the like and heat buildup within the case. As a result, reliability of the control unit may decrease.